Add `ContentNotFoundError`

[acolytec3]

In the process of researching Ultralight test failures for Hive, I discovered that the JSON RPC spec currently advises that the portal_*localContent methods return 0x when no content is found. However, this did not solve a hive test failure for portal_beaconLocalContent for Ultralight when I updated our code to match this spec. After further research, I discovered that trin and fluffy return an error and not 0x when content is not found. This PR updates the spec to reflect the current implemented state of the clients.

Specifically:

• Corrects the portal_*localContent responses to reflect that the clients return an error
• Adds a new ContentNotFoundError a specifies that the error code returned should be -32001 for the ContentNotFoundError and the message "content not found" following the current Trin implementation.
• Adds a -32002 for the ContentNotFoundErrorWithTrace error type that is returned by the traceRecursiveFindContent method types

[acolytec3]

@ogenev @kdeme @KolbyML any outstanding thoughts or are you good with merging this?